Zeolite composite membranes obtained by forming a zeolite membrane on a substrate have been known and have been used as a gas separation membrane or a liquid separation membrane. With respect to such zeolite composite membranes, it is known that when the SiO2/Al2O3 (molar ratio) of the membrane (hereinafter, it means a molar ratio when simply “SiO2/Al2O3” is mentioned) varies, the interaction between the pore surface of zeolite and a molecule passing therethrough varies and the characteristic of the membrane when used as a separation membrane varies as well.
For example, p. 25 of an article by Yoji Sano and Yushi Kawakami in Kagaku Kogyo, February 1995 discloses a finding on the influence of SiO2/Al2O3 on the permeation and separation characteristic of MFI type zeolite membrane (hereinafter referred to also as “MFI membrane”) when water and alcohol are separated from each other by pervaporation using the membrane. It is known that the membrane shows striking alcohol selectivity when the SiO2/Al2O3 thereof is increased.
As a specific example of the generally used MFI membrane, there can be mentioned a zeolite composite membrane obtained by forming a MFI membrane on an alumina substrate. With respect to such a zeolite composite membrane, it is known that, during the formation of the MFI membrane, aluminum in the alumina-made substrate dissolves into the MFI membrane and is taken into the skeleton of the MFI membrane and, as a result, the MFI membrane becomes a SiO2/Al2O3— reduced MFI membrane (hereinafter this membrane is expressed also as “low-silica MFI membrane”).
Also, JP-A-6-127937 discloses a self-supported MFI membrane not formed on any substrate (hereinafter this membrane is referred to as a “MFI self-supported membrane”), into which aluminum is taken and wherein the SiO2/Al2O3 is reduced, and a method for the production thereof.
In the zeolite composite membrane obtained by forming a MFI membrane on an alumina substrate, however, it is impossible to strictly control the SiO2/Al2O3 of the MFI membrane and, therefore, it is difficult to steadily synthesize a low-silica MFI membrane exhibiting uniform separation characteristic. Further, in synthesis of the MFI membrane, a structure-directing agent is added generally and it is necessary to remove the structure-directing agent by high-temperature calcination; however, since the substrate alumina and the MFI membrane (zeolite) differ in thermal expansion coefficient, the MFI membrane may generate cracks during calcination.
In the zeolite membrane described in JP-A-6-127937, since it is a MFI self-supported membrane, problems such as crack generation in MFI membrane due to difference in thermal expansion coefficient between substrate and zeolite can be avoided. Even in the method for the production of MFI membrane according to the above literature, however, it is described therein that the MFI membrane formed at the initial stage of synthesis has a large SiO2/Al2O3 and the SiO2/Al2O3 tends to become smaller with the growth of the membrane; therefore, a low-silica MFI membrane having small value in SiO2/Al2O3 is obtainable only when the thickness of the membrane is made large. Consequently, the MFI membrane obtained exhibits permeation and separation characteristic as a low-silica MFI membrane, but has a large thickness and accordingly a small permeation factor and has a problem of low permeability.
The present invention has been made in view of such problems possessed by the prior art, and aims at providing a laminated zeolite composite having high separation characteristics and a high permeability, and a method for producing such a laminated zeolite composite.